(a) Field of the Invention:
The present invention relates to a photo eyepiece for endoscopes which is usable for both observation by naked eyes and photographing with a camera attached on the side of said eyepiece.
(b) Description of the Prior Art:
In observation through an endoscope, diopter is generally adjusted for matching with observer's visual acuity by displacing the eyepiece along the optical axis. For photographing an image transmitted through an image transmitting optical system with a photographic camera attached to the eyepiece, it must always be set at a pre-determined position since it serves as a photographing lens. For this purpose, the eyepiece is displaced toward the rear side until it is set at the pre-determined position for photographing. When a photo eyepiece is used as an eyepiece, an observer is likely to keep his eye rather apart from the eyepiece, and an image formed with a bundle of optical fibers is dark on its circumferential portions since said bundle has such a characteristic as to make light intensity distribution high at the central portion where rays are almost parallel. Therefore, a photo eyepiece must be so designed as to have a large numerical aperture and, at the same time, be free from aggravation of flare due to coma caused by such a large numerical aperture.
Though optical fibers conventionally had large diameters, they have recently been gradually thinned for asuring high image resolution on end surfaces of optical fiber bundles. It is therefore required to assure minimum aberrations over the entire range of an image formed with a photo eyepiece, especially by favorably correcting spherical aberration, lateral chromatic aberration, and further curvature of field so favorably as to obtain a highly flat image. Further, since rays are diversed while transmitting through an image transmitting optical system of an endoscope, an eyepiece therefor must have a numerical aperture several times as large as that of an objective lens. The eyepiece will have a narrow depth of focus accordingly.
Through the defect due to such a narrow depth of focus can be compensated by the adjusting function of observer's eye in observation, said defect poses a problem in photographing and, therefore, curvature of field must be corrected more favorably.